In measuring the volume and/or rate of flow of fluids, a vaned rotor is commonly mounted in the flow passage, designed to rotate accurately in proportion to the volume of flow of fluid, especially liquid, over a wide range of flow rates. Where the rotor is used to provide mechanical drive force to operate the gearing of a register, the resulting loading of the rotor introduces inaccuracy that is particularly notable at low flow rates. Rather than to rely on the rotor to provide mechanical drive for a register, sensors have been located close to the rotor to generate flow-representing signals. For example, magnets have been mounted on the flow-metering rotor, arranged to induce pulses in a sensing coil that is mounted in a plug sealed in the wall of the pipe, close to the rotor magnets. Electrodes exposed to water adjacent a turbine rotor provide flow sensing in copending application Ser. No. 06/150,142, filed May 15, 1980, by the present inventor. These and other flow sensors are known.
A so-called turbine rotor is commonly used as the metering element in the flow passage. The rotor axis may be centered in a pipe, or the turbine rotor may be mounted as a probe, having a rotor of small diameter in a much larger-diameter passage, with the rotor axis parallel to the direction of flow. Probe-type metering elements of the "paddle-wheel" type are also known, in which the rotor has blades coplanar with the rotor axis at right angles to the direction of flow. Wobble-type flow metering elements are also known. In each instance, the metering element operates in cycles to represent, ideally, a metered volume of flow per cycle. Metering elements characteristically have a vane or other discrete portion that moves cyclically past or opposite to a spot where a magnetic pick-up or other sensor may be located so as to yield one pulse or wave each time a discrete portion of the metering element passes the sensor.
Other types of flow sensors are known which depend on an interaction between a sensor and anomalies in the fluid. One such flow sensor responds to the interaction between the liquid in a passage and a supersonic acoustic signal source. Those flow sensors omit the flow-metering element in the other flow sensors that are briefly described above.